One of the characteristics which determine the quality of a loudspeaker is the directivity. The directivity defines variations of the sound pressure in different propagating directions of the sound. However, a wider directivity does not automatically ensure the quality of the speaker. It is rather advisable to determine the directivity pattern depending on the purpose of the speaker and the size of the area where the loudspeaker is expected to carry sound. For example, for an audio system, a wide directivity is required. For a public-address system, in order to prevent howling, a narrow directivity wherein the sound is propagated only in certain directions is required.
There are other factors to be considered when determining the directivity of the loudspeaker. In a speaker system employing a single speaker unit, the directivity is determined depending on the construction of the unit, that is, whether the speaker unit is a cone speaker or a horn speaker, and in the case of the cone speaker, the depth of the conical diaphragm. In a line source speaker system, where a plurality of speaker units are disposed in a linear array, each speaker unit is adapted to emit sound only in a direction determined in accordance with the physical construction and disposition of the speaker units.
Japanese Patent Application Laid-Open Nos. 2-239798 and 5-41897 disclose systems where digital filters are provided to electrically control a directivity pattern.
Referring to FIG. 6, such a speaker system comprises a digital filter array 10, amplifier array 12, and a speaker unit array 14. The digital filter array 10 comprises digital filters 16-1 to 16-n each of which is a finite impulse response (FIR) filter. Each digital filter is applied with the same input signal through a common input terminal IN. The amplifier array 12 comprises amplifiers 18-1 to 18-n, each of which is connected with the corresponding filters 16-1 to 16-n, and further to corresponding speaker units 20-1 to 20-n of the speaker unit array 14. The filter coefficients of each digital filter is set such that each speaker unit has a directivity adapted to provide an optimum directivity as a whole system.